"Beside myself with joy"

It means experiencing extreme joy. "Beside" was formerly (15th
through 19th centuries) used in phrases to mean "out of a mental
state or condition, as 'beside one's patience, one's gravity, one's
wits'" (Oxford Engl. Dict.), and that use survives only in "'beside
oneself': out of one's wits, out of one's senses.""